The coating of metallic substrates with an unbonded ply of plasticized thermoplastic resin has been well established commercially heretofore. The incorporation in the resin coating compositions of various standard anti-oxidants, light stabilizers and other conventional additives has resulted in coated metallic materials manifesting a flexibility without cracking, an impact hardness and resistance to abrasion which makes them particularly useful in a variety of applications including chain-link fence.
The metallic substrate of these coated materials is rendered vulnerable however, where a single-ply of unbonded plastic is present, because of the relative ease with which the resin coating can be stripped from the substrate, a particular concern, for example, where the coated material is chain-link fence and where this material is used in areas subject to the activity of vandals, such as heavily industrialized locations, public playgrounds and the like.
The bonding of certain thermoplastic resin coatings to a metallic substrate has been known to reduce this ease of removal. Bonding has been accomplished, illustratively, by treating wire, for example, with a primer heated to an elevated temperature and the composite of wire and primer passed through a fluidized bed of vinyl resin powder. The wire substrate used commercially in the practice of this latter process has been found to be ungalvanized steel. Inherent in this process, additionally, has been the formation of a microporous coating of limited thickness, i.e., about 7-10 mils, and this vinyl coating has been found to evidence reduced resistance to ultra-violet radiation over a sustained period. The relative thinness of the coating which can be achieved by this method has been found to permit corrosive atmospheres even in the absence of removal of the coating. This vulnerability is, of course, of particular significance where the substrate is, illustratively, ungalvanized steel. The production of vinyl coated metallic substrates employing plastisols or organosols of vinyl chloride resins has also been projected, but the combination of steps including particularly the removal of diluents from the coating and the absorption of plasticizers in the fusion phase tend to render the processes uneconomic, both by reason of the reduced speeds at which, for example, wire must pass through the coating step, e.g., up to about 300 feet per minute, and the high temperature baking ovens necessary for fusion, utilizing high levels of electric energy.
Securing a plastic composition to a metal element is disclosed specifically and by way of further illustration in U.S. Pat. No. 3,795,540. The bonding of an extruded plastic cover of polyvinyl chloride, rubber, impregnated paper or preferably polyethylene, for example, is suggested by this reference using a copolymer of ethylene and an ethylenically unsaturated carboxylic acid, particularly ethylene-vinyl acetate copolymer. This reference is not concerned with a product capable of being produced at high speeds in a continuous process and incorporating a significantly superior bond of coating to substrate. The formation of an adhesive-coated substrate and a substrate to which the polyethylene polymer is thereupon applied is undertaken under inherently slow moving conditions in which the adhesive must be extruded into the substrate. Thus, the adhesive employed provides a bond between a protective polymeric coating such as poly (vinyl chloride) and a metallic substrate which is inadequate particularly for high-speed metal forming operations; for example, the production of wire products such as chain-link fence.
A further method suggested heretofore for producing a metal component coated with a bonded plastic composition is that described in U.S. Pat. No. 2,531,169 wherein the patentee describes the deposition upon wire of a phenolaldehyde modified polyvinyl enamel, a thermoset lacquer, as an adhesive, with sequential baking, and, in order to secure the necessary thickness, passing the wire through the enamelling bath and baking oven a number of times, after which the enamelled wire is transmitted through a vinyl dispersion or plastisol with heating of the latter coating as well. This latter coating step is also repeated several times. This method is obviously cumbersome and uneconomic. This patent suggests that extrusion techniques are unsuitable for deposition of thin plies of plastic material because of the tendency to damage the undercoat previously placed on the substrate and because of nonuniformity in the resulting layer.
Certain of these disadvantages elucidated, illustratively, in the disclosure of U.S. Pat. No. 2,531,169 are apparent in U.S. Pat. No. 3,532,783 wherein a polyethylene coating is attached by means of a high density polyethylene modified with maleic acid to a wire substrate. This latter patent suggests that polyvinyl chloride may be substituted for polyethylene if a suitable adhesive can be found. The adhesive suggested is VMCH, a vinyl chloride-vinyl acetate copolymer that is deposited only from solution. Under normal application this vinyl composition is air dried or baked to eliminate residual solvents. However, even if force dried, the desired state for application will be effected only very slowly. Once deposited, in any event, on the metal substrate with a subsequent overcoat layer of plasticized vinyl compound, the adhesive is softened by the plasticizer of the vinyl chloride resulting in poor bond strength. The process described in this patent proceeds inherently at a slow pace because of the necessity to heat the wire substrate that is to be coated in order to effect a proper deposition of adhesive. The solid flake adhesive employed, in addition, presents a material problem in securing a uniform coat, enhancing the dependency of the process on the preheating step.
If, accordingly, a product could be devised comprising a metallic substrate, and particularly wire, and, as a second layer or ply, a hot melt polyamide adhesive capable of bonding firmly the wire and a further ply of extrudable thermoplastic resin and particularly polyvinyl chloride or copolymers thereof having a uniform thickness sufficient to provide effective and prolonged protection to the wire substrate, a product of prolonged life span would be obtainable, reducing, and indeed, substantially eliminating the replacement now periodically required of materials which are increasingly expensive or unavailable, and thus constitute a significant advance in the state of the art. Similarly, if an economically and technically feasible, continuous, high speed system of providing a product such as the foregoing wherein the heat absorbing qualities of the metallic substrate are used to cool the adhesive could be devised, an advance of significant merit would also be effected.
Various polyamide adhesives have been proposed generally for use with polyvinyl chloride and with metals but no mode of application, much less one that is economically efficacious, or capable of uniform and continuous performance at high speeds; nor indeed any suggestion as to specific adhesives appropriate for simultaneous application to metals and polyvinyl chloride to secure a permanent bond is apparent in these teachings.